Abnormalities in magnesium metabolism have been identified in women with the Premenstrual Syndrome (PMS). We evaluated plasma, red blood cell and mononuclear blood cell magnesium concentration across the menstrual cycle in 26 women with PMS and 19 control women. Data were analyzed using analysis of variants with repeated measures and Spearman rank correlations. The results showed a significant decrease in red blood cell and mononuclear blood cell magnesium concentrations for women with PMS at each sampling time compared to controls. These deficits were not confined to the luteal phase but were present throughout the menstrual cycle. Further, we evaluated blood magnesium parameters and the plasma calcium to magnesium ratio in a double-blind placebo-controlled study of methylphenidate and dextroamphetamine in hyperactive boys. Plasma magnesium was significantly higher after 3 weeks of dextroamphetamine and the calcium-magnesium ratio was significantly lower after 3 weeks of either drug compared to baseline or placebo. The decrease in the calcium-magnesium ratio may be relevant to our understanding of side effects and "treatment resistance" associated with stimulant use.